Share a specific file from server side

Currently, you can only receive a file from anyone but the objective of this feature is ;
- To add a dash feature which allows you to share any specific file with others.

The issue describes adding a feature to share a specific file from the server side. Look at the existing file-sharing logic in the Flask backend and the CLI client. Determine how file selection and sharing permissions are currently handled. The goal is to allow a user to specify a file to share with others, which likely involves modifying both server endpoints and client commands.

索引モデルが issue の本文から書いたものです。